In a radio networking environment that re-uses frequencies (e.g., a cellular telephony and data network), a mobile device often simultaneously receives signals from multiple transmitters even though the mobile device is normally only in active communications with one transmitter at a time. The mobile device monitors the strength of these signals. When the strength of a signal becomes greater than the strength of the signal received from the transmitter with which the mobile device is actively communicating, then the mobile device is “handed off” from its current transmitter to the transmitter with the stronger signal. (Note that the strength of the signal as measured by the mobile device typically becomes stronger or weaker as the mobile device moves nearer to, or farther from, respectively, the transmitter.) The mobile device then actively communicates with the new transmitter. The mobile device continues to monitor the strengths of the signals it receives and may be handed off to yet another transmitter.
This hand-off method allows the mobile device to advantageously direct its communications to the transmitter with the strongest signal (which is usually the nearest transmitter). The method also provides the benefit of reduced interference. When the mobile device receives signals from multiple transmitters, the signals from all transmitters other than the one with which the mobile device is actively communicating can interfere with the transmissions between the mobile device and its chosen transmitter. The amount of interference grows as the strength of the interfering signal becomes greater (e.g., as the mobile device moves closer to the interfering transmitter). In the hand-off method described above, interference problems are resolved because the mobile device eventually switches to the transmitter with the strongest signal. Thus, what was once the greatest source of interference becomes the mobile device's communications partner, eliminating this transmitter as a source of interference.
This hand-off method provides the interference-reduction benefits only if the communications network permits the mobile device to be handed off to the transmitter with the strongest signal. While this is generally the case, it may not be the case when a transmitter's signal overlaps with the signal from a “private” transmitter. For purposes of the present discussion, we use the terminology of cellular networks: The “normal” or non-private transmitters are called “macrocell” transmitters, and the private transmitter is called a “femtocell” transmitter. The provider of the femtocell may secure it so that the femtocell does not allow itself to become the active transmitter for every mobile device that comes along. (Often, this means that the femtocell will only host those mobile devices that know how to log into the femtocell using well known authentication and security protocols.) Although femtocells usually transmit at much lower power levels than used by macrocell transmitters, when a mobile device moves “close enough” to the femtocell transmitter, the signal of the femtocell can become a source of interference that current hand-off methods cannot address. In some cases, the interference is so bad that it creates a “coverage hole” in the macrocell.
Repeating the above example with a little more detail, a mobile device moves toward a secured femtocell transmitter until the femtocell's signal interferes with (or potentially interferes with) the mobile device's communications with a macro cell transmitter. This particular mobile device is not authorized to access the femtocell and thus cannot be handed off to the femtocell. The mobile device must continue to communicate with its macrocell transmitter even though the signal received from the macro cell may be weaker than the signal received from the closer femtocell. The interference from the femtocell grows and cannot be relieved by current methods. As a corollary, the transmissions between the mobile device and its macro cell may interfere with transmissions between the femtocell and its devices.